Ceramic materials have gained the attention of industry by virtue of their low cost and superior performance qualities. These qualities, such as superior high temperature strength, high toughness, resistance to thermal shock, and resistance to oxidation provide the bases for their potential use in a variety of applications.
Although complex-shaped ceramics have been considered in many applications, spherical ceramics appear to be especially promising. For example, silicon nitride is presently widely used in ball bearings. These bearings can be used without lubrication and so provide an advantage over conventional metal bearings which require lubrication. Further, silicon nitride is also a candidate milling medium for ball-milling operations. Because ball-milling is a high-wear application, the toughness and hardness of silicon nitride make it highly desirable for this use.
The production of a ceramic ball bearing or milling ball requires the formation of an unsintered, moderately dense, spherical green body which can then be sintered into a hard, dense ceramic ball. However, conventional methods of forming green bodies appear to have limitations which render them unsuitable for forming the desired spherical bodies. For example, slip casting appears to be impractical for the large scale production of a simple shape. Uniaxial die pressing has been used to produce ceramic balls having very good dimensional control. However, this process requires the use of an expensive die press, and produces a ceramic ball having undesirable gradients within the green body and an undesirable mold flashing, or lip, upon the surface of the green body. This lip must be removed from the green body in order to meet tolerance requirements and is often machined off in a tumbler. Injection molding processes offer an alternative method of making the green sphere. However, it requires expensive molds and a long dewaxing process to remove the binder prior to sintering. Various granulation methods also produce spherical balls by the snow-balling approach, wherein a seed is rolled in a powder. However, substantial radial gradients are observed and dimensional control by this method is less than desirable.
Because the conventional methods of forming green bodies have proven unsatisfactory for producing spherical shapes, the art has focused on alternative methods of making such spheres. For example, European Patent Publication No. 434,164 (Contursi) teaches forming a mixture comprising a ceramic powder precursor, carbon and a carbohydrate, dropping the mixture into an alkaline solution thus converting it to a sol, and nitriding the spherical sols to form a powder. The process does not produce spherical green bodies and requires a time-consuming chemical reaction between its strongly alkaline solution (7M NH.sub.4 OH) and its mixture in order to form its sol.
U.S. Pat. No. 4,441,905 (Malmendler) describes a method of making spherical bodies comprising: a) releasing gel droplets containing crystals of synthetic mica into a liquid having a surface tension that tends to spheroidize the droplets, b) solidifying the bodies via cation exchange within an ion exchange medium; and, c) separating the bodies from the liquid medium. Although Malmender's method of gel solidification does produce spheroids, it also requires the use of crystals of synthetic mica and ion exchange, and so is a complicated process of narrow applicability.
U.S. Pat. No. 4,865,829 (Hattori) describes a) suspending a silicic acid ester/water mixture in a medium comprising ethyl cellulose and an organic liquid which is immiscible in the mixture, b) hydrolyzing and polymerizing the mixture to produce a spheroid gel particle dispersion slurry, c) separating the gel particles from the dispersing medium, and d) drying the spheres. However, not only are undesirably long reaction times and high temperatures needed to polymerize the ester, the method appears to be restricted to production of relatively porous, silica green bodies.
U.S. Pat. No. 5,030,391 (Sumita) describes a method of making spherical bodies comprising (i) pouring a ceramic/water mixture into an organic liquid to form a water-in-oil slurry, (ii) pouring the water-in-oil slurry into water to form a water-in-oil-in-water ("w/o/w") slurry having a polymer-coated ceramic phase, (iii) heating the polymer to form a solid polymer shell and a ceramic core (iv) separating the shell and core from the aqueous solution, and (v) removing the shell by thermal decomposition. Clearly, this method is quite complicated, requiring a w/o/w slurry formation step and two heating steps to obtain a porous ceramic green body.
U.S. Pat. No. 4,734,237 (Fanelli et al.) teaches a method of forming green bodies comprising the steps of: (i) mixing a ceramic powder and an agaroid gel-forming material into a solvent in which the agaroid is soluble, (ii) supplying the mixture into a mold, and (iii) forming a molded green body by lowering the temperature of the mixture to a point below the gel-forming temperature of the agaroid material. Although the Fanelli et al. process overcomes some of the shortcomings of the above-mentioned gel-forming art, it does not teach producing spheroid green bodies. Further, the green bodies it does produce have low densities which are extremely susceptible to deformation during handling and, when sintered, yield ceramic bodies having theoretical densities of only about 80%.
Matthews and Simpson, Ceramic Bulletin, Vol. 58, No. 2, pp. 223-227 (1979), disclose dropping ceramic precursor sols into a column of 2-ethyl hexanol to form a sphere. Water is slowly extracted from the sol by the 2-ethyl hexanol, and the sol transforms chemically into a gel. As the gelled sphere densifies, it drops out of the column. However, not only is this method limited to having sols as starting materials, but also the chemical transformation of the sol to a gel requires a long reaction time.
Thus, it is the object of the present invention to provide an improved method of making green bodies from various metal and ceramic powders which overcomes shortcomings of the prior art.